The invention lies in the semiconductor technology and lithography field and relates, more specifically, to an EUV reflection mask and a fabrication method therefor.
In EUV technology, a layer to be exposed which lies on a semiconductor wafer is exposed with EUV radiation, i.e. radiation in the extreme ultraviolet spectral range, by means of an EUV reflection mask. The exposure radiation thereby impinges on the mask not perpendicularly but at a small angle of incidence relative to the solder, is reflected from reflecting regions of the mask and then falls onto the light-sensitive layer of the wafer. This small angle of incidence is due to the fact that the exposure apparatuses that are customary nowadays do not provide central symmetrical guidance.
A fabrication method that has been customary heretofore for an EUV reflection mask and a mask thereby fabricated are explained below with reference to the FIGS. 3 and 4. FIG. 3 shows a mask blank 100 comprising a substrate 101, serving as a carrier support, an overlying multilayer layer 102 made of molybdenum and silicon layers, an optional top layer 103, which lies above the multilayer layer 102 and serves as oxidation protection a likewise optional buffer layer 104, an absorber layer 105, which has the property of absorbing the exposure radiation and is composed of chromium, for example, and a topmost resist layer 106. For mask fabrication, a desired exposure pattern is then written into the resist layer 106, the resist layer 106 is developed and the developed regions are etched to form pattern structures.
This method produces the EUV reflection mask 110 shown in FIG. 4. It is clear that the absorbing regions A form elevated structures above the multilayer layer 102 or the overlying top layer 103, and that the reflecting regions R lie recessed. The exposure radiation, i.e. EUV light, indicated by dashed arrows S must be directed, as mentioned, onto the mask surface at a small angle a. (The angle a is drawn in exaggerated fashion.) In this case, shadow casting arises at the edges of the projecting absorbing pattern regions A. Since this shadow casting is problematic for complying with precise exposure in particular of very small structures, it is desirable to avoid this shadow casting, or. reduce it to a minimum. At the present time, attempts are being made to reduce or eliminate the shadow casting by setting a specific angle of the walls of the absorbing structures A. A further attempt to fill the interspaces between the absorbing regions A with material of the multilayer layer 102 has not been successful heretofore.
This method produces the EUV reflection mask 110 shown in FIG. 4. It is clear that the absorbing regions A form elevated structures above the multilayer layer 102 or the overlying top layer 103, and that the reflecting regions R lie recessed. The exposure radiation, i.e. EVU light, indicated by dashed arrows S must be directed, as mentioned, onto the mask surface at a small angle a. (The angle a is drawn in exaggerated fashion.) In this case, shadow casting arises at the edges of the projecting absorbing pattern regions A. Since this shadow casting is problematic for complying with precise exposure in particular of very small structures, it is desirable to avoid this shadow casting, or reduce it to a minimum. At the present time, attempts are being made to reduce or eliminate the shadow casting by setting a specific angle of the walls of the absorbing structures A. A further attempt to fill the interspaces between the absorbing regions A with material of the multilayer layer 102 has not been successful heretofore.
It is an object of the invention, therefore, to specify a fabrication method for an EUV reflection mask and a reflection mask of this type that avoid the disadvantages of the prior art devices of the generic kind and which, specifically, avoid the problem of shadow casting. In this case, in an advantageous manner, lithography in the conventional sense will no longer be used in the mask fabrication, rather the mask will be produced in a single step.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of fabricating an EUV reflection mask for the region-by-region exposure of a radiation-sensitive layer on a semiconductor wafer, which comprises:
providing a substrate with a planar multilayer layer on a front side thereof;
carrying out a writing step to directly reduce a reflectivity of portions of the planar multilayer layer on the substrate to form radiation-absorbing mask regions and to form radiation-reflecting mask regions.
In accordance with an added feature of the invention, the multilayer layer comprises molybdenum and silicon layers.
In accordance with an additional feature of the invention, the writing step comprises irradiating with laser radiation to reduce the reflectivity of the radiation-absorbing mask regions. In the alternative, the writing step comprises reducing the reflectivity of the radiation-absorbing mask regions through ion implantation. Preferably, the ions are implanted into the radiation-absorbing mask regions by way of focused ion radiation.
A semiconductor can therefore be exposed by first forming a mask as outlined above and irradiating with radiation in a spectral range of extreme ultraviolet radiation to reflect with reflecting mask regions and to allow radiation to traverse the radiation-absorbing regions to expose the wafer in accordance with a pattern to be exposed on the semiconductor wafer.
With the above and other objects in view there is also provided, in accordance with the invention, an EUV reflection mask for region-by-region exposure of a radiation-sensitive layer on a semiconductor wafer with radiation in a spectral range of extreme ultraviolet radiation, comprising:
a multilayer layer with a planar surface on a front side of a substrate, said multilayer layer having sections with an altered material constitution or structure defining pattern regions configured to reflect EUV radiation and radiation-absorbing regions at a front side of the mask facing toward the semiconductor wafer to be exposed, said pattern regions and said radiation-absorbing regions corresponding to patterns to be formed on the semiconductor wafer;
wherein the material constitution or structure of the multilayer layer is altered in sections such that the radiation-absorbing pattern regions have a greatly reduced reflectivity compared with the radiation-reflecting regions.
In accordance with a concomitant feature of the invention, at least some layers of the multilayer layer are fused together at the regions of reduced reflectivity of the mask.
The inventors have recognized that it is possible to write the absorbing pattern structures directly into the multilayer layer 102. Through a suitable method, it is possible to reduce the reflectivity at the locations which are intended to absorb the exposure radiation, i.e. at the absorbing mask structures, either by layers of the multilayer layer 102 being fused together, or by impurity atoms being implanted into said absorbing regions by ion bombardment.
In an advantageous manner, the fabrication method according to the invention not only achieves a simplification of the process but also avoids the problem of shadowing, since the surface is planar and absorbing structures projecting from the surface of the mask no longer exist. The fabrication method according to the invention can achieve a difference in reflectivity between the radiation-reflecting regions and the radiation-absorbing regions in the multilayer layer of at least 90%.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a EUV Reflection Mask, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.